The present invention relates to pulverizers in which materials such as coal are pulverized to a required fineness between relatively moving grinding surfaces and, more particularly, to a roll-and-race type pulverizer that is provided with a rotating throat and improved throat design.
Pulverizers utilize crushing pressure, impact and attrition between grinding surfaces to reduce materials to a fine particle size. Roll-and-race pulverizers are among the several types of pulverizer designs that are commonly employed, particularly to prepare coal for the firing of boilers. In roll-and-race pulverizing, the grinding assemblies essentially comprise rolls mounted within grinding rings. In the so-called standard roll-and race pulverizer, the ring is stationary and the rolls rotate. In the so-called bowl mill roll-and-race pulverizer, the rolls are stationary and the ring rotates.
The principal components of a bowl mill are a rotating bowl which is equipped with a replaceable grinding ring, a plurality of rolls mounted in stationary journals, an automatically controlled feeder, a classifer, and a drive. The rolls in the bowl mill are resiliently urged against the grinding ring by springs which apply force to the axles of the rolls. Raw coal fed to the pulverizer mixes with partially ground coal that is circulated within the grinding zone by a stream of air which flows through the pulverizer. As the coal is reduced in size, it is fluidized by the air and carried to the classifier. The properly-sized pulverized coal is pneumatically conveyed from the pulverizer through outlet pipes. Oversized coal is returned to the grinding zone.
The carrier air generally passes upwardly from a plenum through an annular throat and picks up the ground coal along the outer edge of the grinding ring. The throat is divided into a plurality of circumferential segments by vanes that radially extend between an inner circular throat wall and an outer circular throat wall. Excessive air pressure losses, the non-uniform distribution of air within the segments and the dribble of materials downwardly through the throat against the ascending stream of air have been persistent problems in the art of pulverizer design.
Throughout the years, many designs have been proposed in an effort to resolve such problems. See, for example, U.S. Pat. Nos. 2,275,595, 2,378,681, 2,389,844, 2,431,746, 2,473,514, 2,545,254, 2,698,142, 4,264,041 and 4,523,721.
U.S. Pat. No. 4,264,041, which is assigned to the assignee of the present invention, discloses an improved stationary vane configuration. The stationary vanes extend over the radial width of the throat and are designed with a tear-drop and air foil vane shape to provide an acceleration of carrier air flow rates within the throat in order to produce a more uniform throat velocity while maintaining threshold velocities needed to preclude dribble of the pulverized material. Although representing an improvement, tests of coal pulverizers, employing the stationary vane design of U.S. Pat. No. 4,264,041, reveal that air flow rates vary as much as fifteen percent from the average with no coal in the pulverizer and as much as twenty-five percent from the average when coal is present. Consequently, the large variation in flow rates has dictated the need to use greater overall flows to be sure that minimum threshold velocities are achieved. The additional overall flow requires additional fan power consumption. In addition, variations in the flow rates create a non-uniform flow of coal off of the grinding ring and results in uneven throat wear and vane erosion since certain throat openings or vanes are being subjected to higher or lower than average velocities.